Overvoltage protection device including multiple varistor wafers

ABSTRACT

An overvoltage protection device includes a first electrode member, a second electrode member, and a varistor assembly. The varistor assembly includes: a plurality of varistor wafers each formed of a varistor material; and at least one electrically conductive interconnect member connecting the varistor wafers in electrical parallel between the first and second electrode members. The varistor wafers are axially stacked between the first and second electrodes.

RELATED APPLICATION(S)

The present application is a continuation of and claims priority fromU.S. patent application Ser. No. 15/795,986, filed on Oct. 27, 2017,which is a continuation-in-part of and claims priority from U.S. patentapplication Ser. No. 15/389,870, filed on Dec. 23, 2016, now U.S. Pat.No. 10,447,026, the entire contents of which are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates to circuit protection devices and, moreparticularly, to overvoltage protection devices and methods.

BACKGROUND

Frequently, excessive voltage or current is applied across service linesthat deliver power to residences and commercial and institutionalfacilities. Such excess voltage or current spikes (transientovervoltages and surge currents) may result from lightning strikes, forexample. The above events may be of particular concern intelecommunications distribution centers, hospitals and other facilitieswhere equipment damage caused by overvoltages and/or current surges andresulting down time may be very costly.

Typically, sensitive electronic equipment may be protected againsttransient overvoltages and surge currents using Surge Protective Devices(SPDs). For example, brief reference is made to FIG. 1, which is asystem including conventional overvoltage and surge protection. Anovervoltage protection device 12 may be installed at a power input ofequipment to be protected 50, which is typically protected againstovercurrents when it fails. Typical failure mode of an SPD is a shortcircuit. The overcurrent protection typically employed is a combinationof an internal thermal disconnector to protect the device fromoverheating due to increased leakage currents and an external fuse toprotect the device from higher fault currents. Different SPDtechnologies may avoid the use of the internal thermal disconnectorbecause, in the event of failure, they change their operation mode to alow ohmic resistance. In this manner, the device can withstandsignificant short circuit currents. In this regard, there may be nooperational need for an internal thermal disconnector. Further to theabove, some embodiments that exhibit even higher short circuit withstandcapabilities may also be protected only by the main circuit breaker ofthe installation without the need for a dedicated branch fuse.

Brief reference is now made to FIG. 2, which is a block diagram of asystem including conventional surge protection. As illustrated, a threephase line may be connected to and supply electrical energy to one ormore transformers 66, which may in turn supply three phase electricalpower to a main circuit breaker 68. The three phase electrical power maybe provided to one or more distribution panels 62. As illustrated, thethree voltage lines of the three phase electrical power may designatedas L1, L2 and L3 and a neutral line may be designated as N. In someembodiments, the neutral line N may be conductively coupled to an earthground.

Some embodiments include surge protective devices (SPDs) 15. Asillustrated, each of the SPDs 15 may be connected between respectiveones of L1, L2 and L3, and neutral (N). The SPD 15 may protect otherequipment in the installation such as the distribution panel amongothers. In addition, the SPDs may be used to protect all equipment incase of prolonged overvoltages. However, such a condition may force theSPD to conduct a limited current for a prolonged period of time, whichmay result in the overheating of the SPD and possibly its failure(depending on the energy withstand capabilities the SPD can absorb andthe level and duration of the overvoltage condition). A typicaloperating voltage of an SPD 15 in the present example may be about 400V(for 690V L-L systems). In this regard, the SPDs 15 will each perform asan insulator and thus not conduct current during normal operatingconditions. In some embodiments, the operating voltage of the SPD's 15is sufficiently higher than the normal line-to-neutral voltage to ensurethat the SPD 15 will continue to perform as an insulator even in casesin which the system voltage increases due to overvoltage conditions thatmight arise as a result of a loss of neutral or other power systemissues.

In the event of a surge current in, for example, L1, protection of powersystem load devices may necessitate providing a current path to groundfor the excess current of the surge current. The surge current maygenerate a transient overvoltage between L1 and N. Since the transientovervoltage significantly exceeds that operating voltage of SPD 15, theSPD 15 will become conductive, allowing the excess current to flow fromL1 through SPD 15 to the neutral N. Once the surge current has beenconducted to N, the overvoltage condition ends and the SPD 15 may becomenon-conducting again. However, in some cases, one or more SPD's 15 maybegin to allow a leakage current to be conducted even at voltages thatare lower that the operating voltage of the SPD's 15. Such conditionsmay occur in the case of an SPD deteriorating.

As provided above, devices for protecting equipment from excess voltageor current spikes (transient overvoltages and surge currents) mayinclude including varistors (for example, metal oxide varistors (MOVs)and/or silicon carbide varistors).

SUMMARY

According to embodiments of the invention, an overvoltage protectiondevice includes a first electrode member, a second electrode member, anda varistor assembly. The varistor assembly includes: a plurality ofvaristor wafers each formed of a varistor material; and at least oneelectrically conductive interconnect member connecting the varistorwafers in electrical parallel between the first and second electrodemembers. The varistor wafers are axially stacked between the first andsecond electrodes.

According to some embodiments, the plurality of varistor wafers includesfirst, second and third varistor wafers, and the at least oneinterconnect member includes at least first and second interconnectmembers connecting the varistor wafers in electrical parallel betweenthe first and second electrode members.

In some embodiments, the first interconnect member contacts andelectrically connects each of the first electrode member and the first,second and third varistor wafers, and the second interconnect membercontacts and electrically connects each of the second electrode memberand the first, second and third varistor wafers.

In some embodiments, each of the first, second and third varistor wafersincludes opposed planar contact faces, each of the first and secondinterconnect members includes two spaced apart, planar contact portionsand a bridge portion extending between and electrically connecting thecontact portions, and the contact portions engage the planar contactfaces.

In some embodiments, each contact portion engages at least 40 percent ofeach contact face engaged thereby.

According to some embodiments, each varistor wafer has a thickness inthe range of from about 0.5 mm to 15 mm.

According to some embodiments, each varistor wafer includesmetallization layers forming opposed planar contact faces of thevaristor wafer.

According to some embodiments, the overvoltage protection deviceincludes a bonding agent bonding at least two of the varistor wafers inthe varistor assembly to one another. In some embodiments, the bondingagent is at least one of cyanoacrylate-based adhesive and epoxy-basedadhesive. In some embodiments, the bonding agent is bonded to peripheraledges of the varistor wafers. In some embodiments, the bonding agentincludes a plurality of circumferentially spaced apart bonding agentmasses bonded to the peripheral edges of the varistor wafers.

According to some embodiments, the first electrode includes a housingelectrode including an end wall and an integral sidewall collectivelydefining a cavity, the second electrode extends into the cavity, and thevaristor assembly is disposed in the cavity. In some embodiments, thehousing electrode is unitarily formed of metal. In some embodiments, theovervoltage protection device includes a biasing device applying anaxially compressive load to the varistor assembly.

According to some embodiments, the overvoltage protection deviceincludes a biasing device applying an axially compressive load to thevaristor assembly.

According to some embodiments, the overvoltage protection deviceincludes an electrically conductive meltable member, wherein themeltable member is responsive to heat in the overvoltage protectiondevice to melt and form an electrical short circuit path across thefirst and second electrode members.

In some embodiments, the overvoltage protection device includes a voidfilling member surrounding at least a portion of the varistor assembly,wherein the void filling member is formed of an electrically insulatingmaterial.

In some embodiments, the void filling member includes a receiver recessand a portion of the interconnect member extends outwardly beyond theplurality of varistors and is disposed in the receiver recess.

According to some embodiments, the first electrode includes a housingelectrode including an end wall and an integral sidewall collectivelydefining a chamber, the chamber includes a first subchamber and a secondsubchamber in fluid communication with the first subchamber, themeltable member is disposed in the first subchamber, the varistorassembly is disposed in the second subchamber and a gap volume isdefined between the varistor assembly and the sidewall; and the voidfilling member is disposed in the gap volume to limit a flow of themeltable member into the gap volume.

In some embodiments, the void filling member occupies at least 50percent of the gap volume.

According to some embodiments, the varistor assembly includes aninsulator wafer axially interposed and stacked between at least two ofthe plurality of varistor wafers.

According to some embodiments, the first electrode is a unitary housingelectrode, the housing electrode includes first and second cavities, thevaristor assembly is disposed in the first cavity, and the overvoltageprotection device further includes a second varistor assembly and athird electrode member. The second varistor assembly is disposed in thesecond cavity. The second varistor assembly includes: a plurality ofvaristor wafers each formed of a varistor material; and at least oneelectrically conductive interconnect member. The varistor wafers of thesecond varistor assembly are axially stacked between the housingelectrode and the third electrode. The at least one interconnect memberof the second varistor assembly connects the varistor wafers of thesecond varistor assembly in electrical parallel between the housingelectrode and the third electrode.

According to further embodiments, a varistor assembly includes: aplurality of varistor wafers each formed of a varistor material; atleast one electrically conductive interconnect member connecting thevaristor wafers in electrical parallel; and a bonding agent bonding atleast two of the varistor wafers in the varistor assembly to oneanother. The varistor wafers and the at least one interconnect memberare axially stacked.

In some embodiments, the bonding agent is at least one ofcyanoacrylate-based adhesive and epoxy-based adhesive.

In some embodiments, the bonding agent is bonded to peripheral edges ofthe varistor wafers.

In some embodiments, the bonding agent includes a plurality ofcircumferentially spaced apart bonding agent masses bonded to theperipheral edges of the varistor wafers.

According to method embodiments of the invention, a method for forming avaristor assembly includes: providing a plurality of varistor waferseach formed of a varistor material; providing at least one electricallyconductive interconnect member; axially stacking the varistor wafers andthe at least one interconnect member such that the at least oneinterconnect member connects the varistor wafers in electrical parallel;thereafter applying an axial load to the varistor wafers and the atleast one interconnect member; and thereafter bonding at least two ofthe varistor wafers in the varistor assembly to one another using abonding agent.

According to further embodiments, an overvoltage protection deviceincludes a first electrode member, a second electrode member, avaristor, an electrically conductive meltable member, and a void fillingmember. The varistor is interposed between and electrically connected toeach of the first and second electrodes. The meltable member isresponsive to heat in the overvoltage protection device to melt and forman electrical short circuit path across the first and second electrodemembers. The void filling member surrounds at least a portion of thevaristor. The void filling member is formed of an electricallyinsulating material. The overvoltage protection device includes asidewall defining a chamber, the chamber including a first subchamberand a second subchamber in fluid communication with the firstsubchamber. The meltable member is disposed in the first subchamber. Thevaristor assembly is disposed in the second subchamber and a gap volumeis defined between the varistor assembly and the sidewall. The voidfilling member is disposed in the gap volume to limit a flow of themeltable member into the gap volume.

In some embodiments, the void filling member occupies at least 50percent of the gap volume.

It is noted that aspects of the invention described with respect to oneembodiment, may be incorporated in a different embodiment although notspecifically described relative thereto. That is, all embodiments and/orfeatures of any embodiment can be combined in any way and/orcombination. These and other objects and/or aspects of the presentinvention are explained in detail in the specification set forth below.

BRIEF DRAWING DESCRIPTION

The accompanying figures are included to provide a further understandingof the present invention, and are incorporated in and constitute a partof this specification. The drawings illustrate some embodiments of thepresent invention and, together with the description, serve to explainprinciples of the present invention.

FIG. 1 is a block diagram of a system including conventional surgeprotection.

FIG. 2 is a block diagram of a system including conventional surgeprotection.

FIG. 3 is a perspective view of an overvoltage protection deviceaccording to some embodiments of the invention.

FIG. 4 is an exploded, perspective view of the overvoltage protectiondevice of FIG. 3.

FIG. 5 is a cross-sectional view of the overvoltage protection device ofFIG. 3 taken along the line 5-5 of FIG. 3.

FIG. 6 is a perspective view of a varistor assembly forming a part ofthe overvoltage protection device of FIG. 3.

FIG. 7 is an exploded, perspective view of the varistor assembly of FIG.6.

FIG. 8 is a cross-sectional view of the varistor assembly of FIG. 6taken along the line 8-8 of FIG. 6.

FIG. 9 is a schematic diagram representing an electrical circuit of thevaristor assembly of FIG. 6.

FIG. 10 is a perspective view of an overvoltage protection deviceaccording further embodiments of the invention.

FIG. 11 is an exploded, perspective view of the overvoltage protectiondevice of FIG. 10.

FIG. 12 is a cross-sectional view of the overvoltage protection deviceof FIG. 10 taken along the line 12-12 of FIG. 10.

FIG. 13 is a cross-sectional view of an overvoltage protection deviceaccording further embodiments of the invention.

FIG. 14 is a cross-sectional view of an overvoltage protection deviceaccording further embodiments of the invention.

FIG. 15 is a perspective view of an overvoltage protection deviceaccording further embodiments of the invention.

FIG. 16 is a cross-sectional view of the overvoltage protection deviceof FIG. 15 taken along the line 16-16 of FIG. 15.

FIG. 17 is a cross-sectional view of an overvoltage protection deviceaccording further embodiments of the invention.

FIG. 18 is an exploded, perspective view of the overvoltage protectiondevice of FIG. 17.

FIG. 19 is a cross-sectional view of the overvoltage protection deviceof FIG. 17 taken along the line 19-19 of FIG. 17.

FIG. 20 is a top view of a void filling member forming a part of theovervoltage protection device of FIG. 17.

FIG. 21 is a cross-sectional view of an overvoltage protection deviceaccording further embodiments of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which illustrativeembodiments of the invention are shown. In the drawings, the relativesizes of regions or features may be exaggerated for clarity. Thisinvention may, however, be embodied in many different forms and shouldnot be construed as limited to the embodiments set forth herein; rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art.

It will be understood that when an element is referred to as being“coupled” or “connected” to another element, it can be directly coupledor connected to the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlycoupled” or “directly connected” to another element, there are nointervening elements present. Like numbers refer to like elementsthroughout.

In addition, spatially relative terms, such as “under”, “below”,“lower”, “over”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. For example, if the device inthe figures is turned over, elements described as “under” or “beneath”other elements or features would then be oriented “over” the otherelements or features. Thus, the exemplary term “under” can encompassboth an orientation of over and under. The device may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein interpreted accordingly.

Well-known functions or constructions may not be described in detail forbrevity and/or clarity.

As used herein the expression “and/or” includes any and all combinationsof one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

As used herein, “monolithic” means an object that is a single, unitarypiece formed or composed of a material without joints or seams.

As used herein, the term “wafer” means a substrate having a thicknesswhich is relatively small compared to its diameter, length or widthdimensions.

With reference to FIGS. 1-9, a modular surge protective device (SPD) orovervoltage protection device according to embodiments of the presentinvention is shown therein and designated 100. In accordance with someembodiments, the overvoltage protection device 100 is used as an SPD inan electrical circuit as discussed above. For example, overvoltageprotection devices 100 may be used in place of the SPD 12 in the systemof FIG. 1 or in place of the SPDs 15 in the system of FIG. 2.

The overvoltage protection device 100 is configured as a unit or modulehaving a lengthwise axis A-A (FIG. 5). The overvoltage protection device100 includes a first electrode or housing 122, a piston-shaped secondelectrode 124, four spring washers 128E, a flat washer 128D, aninsulating ring member 128C, two O-rings 130A, 130B, an end cap 128A, aretention clip 128B, a meltable member 132, and an insulator sleeve 134.

The overvoltage protection device 100 further includes a varistorassembly 150 according to embodiments of the present invention. Thevaristor assembly 150 includes a first varistor member 152, a secondvaristor member 154, a third varistor wafer 156, a first internalinterconnect member 160, a second internal interconnect member 162, anda bonding agent 164.

The overvoltage protection device 100 may further include an integralfail-safe mechanism, arrangement, feature or system 102. The fail-safesystem 102 is adapted to prevent or inhibit overheating or thermalrunaway of the overvoltage protection device, as discussed in moredetail below.

The components 122, 124, 128A-C collectively form a housing assembly 121defining a sealed, enclosed chamber 126. The components 122, 124,128A-E, 132 and 150 are disposed axially between the housing 122 and theelectrode 124 along the lengthwise axis A-A, in the enclosed chamber126.

The housing 122 has an end electrode wall 122A and an integralcylindrical sidewall 122B extending from the electrode wall 122A. Thesidewall 122B and the electrode wall 122A form a chamber or cavity 122Ccommunicating with an opening 122D. A threaded post 122E projectsaxially outwardly from the electrode wall 122A.

The electrode wall 122A has an inwardly facing, substantially planarcontact surface 122G. An annular clip slot 12211 is formed in the innersurface of the sidewall 122B. According to some embodiments, the housing122 is formed of aluminum. However, any suitable electrically conductivemetal may be used. According to some embodiments, the housing 122 isunitary and, in some embodiments, monolithic. The housing 122 asillustrated is cylindrically shaped, but may be shaped differently.

The inner electrode 124 has a head 124A disposed in the cavity 122C andan integral shaft 122B that projects outwardly through the opening 122D.

The head 124A has a substantially planar contact surface 124C that facesthe contact surface 122G of the electrode wall 122A. A pair of integral,annular, axially spaced apart flanges 124D extend radially outwardlyfrom the shaft 124B and define an annular, sidewardly opening groove124E therebetween. A threaded bore 124F is formed in the end of theshaft 124B to receive a bolt for securing the electrode 124 to a busbar,for example. An annular, sidewardly opening groove 124G is defined inthe shaft 124B.

According to some embodiments, the electrode 124 is formed of aluminum.However, any suitable electrically conductive metal may be used.According to some embodiments, the electrode 124 is unitary and, in someembodiments, monolithic.

The electrodes 122, 124, the insulating ring 128C and the end cap 128Acollectively define an enclosed chamber 126 containing the meltablemember 132 and the varistor assembly 150.

An annular gap is defined radially between the head 124A and the nearestadjacent surface of the sidewall 122B. According to some embodiments,the gap has a radial width in the range of from about 1 to 15 mm.

The meltable member 132 is annular and is mounted on the electrode 124in the groove 124E. The meltable member 132 is spaced apart from thesidewall 122B a distance sufficient to electrically isolate the meltablemember 132 from the sidewall 122B.

The meltable member 132 is formed of a heat-meltable, electricallyconductive material. According to some embodiments, the meltable member132 is formed of metal. According to some embodiments, the meltablemember 132 is formed of an electrically conductive metal alloy.According to some embodiments, the meltable member 132 is formed of ametal alloy from the group consisting of aluminum alloy, zinc alloy,and/or tin alloy. However, any suitable electrically conductive metalmay be used.

According to some embodiments, the meltable member 132 is selected suchthat its melting point is greater than a prescribed maximum standardoperating temperature. The maximum standard operating temperature may bethe greatest temperature expected in the meltable member 132 duringnormal operation (including handling overvoltage surges within thedesigned for range of the system) but not during operation which, ifleft unchecked, would result in thermal runaway. According to someembodiments, the meltable member 132 is formed of a material having amelting point in the range of from about 80 to 160° C. and, according tosome embodiments, in the range of from about 130 to 150° C. According tosome embodiments, the melting point of the meltable member 132 is atleast 20° C. less than the melting points of the housing 122 and theelectrode 124 and, according to some embodiments, at least 40° C. lessthan the melting points of those components.

According to some embodiments, the meltable member 132 has an electricalconductivity in the range of from about 0.5×10⁶ Siemens/meter (S/m) to4×10⁷ S/m and, according to some embodiments, in the range of from about1×10⁶ S/m to 3×10⁶ S/m.

The three varistor wafers 152, 154, 156 and the two interconnect members160, 162 are axially stacked in the chamber 126 between the electrodehead 124 and the electrode wall 122 and form the varistor assembly 150.The interconnect members 160, 162 electrically interconnect the wafers152, 154, 156 and the electrodes 122, 124 in the manner represented inthe schematic electrical diagram of FIG. 9.

According to some embodiments, each varistor wafer 152, 154, 156 is avaristor wafer (i.e., is wafer- or disk-shaped). In some embodiments,each varistor wafer 152, 154, 156 is circular in shape and has asubstantially uniform thickness. However, varistor wafers 152, 154, 156may be formed in other shapes. The thickness and the diameter of thevaristor wafers 152, 154, 156 will depend on the varistorcharacteristics desired for the particular application.

In some embodiments, each varistor wafer 152, 154, 156 has a diameter D1to thickness T1 ratio of at least 3. In some embodiments, the thicknessT1 (FIG. 8) of each varistor wafer 152, 154, 156 is in the range of fromabout 0.5 to 15 mm. In some embodiments, the diameter D1 (FIG. 8) ofeach varistor wafer 152, 154, 156 is in the range of from about 20 to100 mm.

The varistor wafer 152 has first and second opposed, substantiallyplanar contact surfaces 152U, 152L and a peripheral edge 152E. Thevaristor wafer 154 has first and second opposed, substantially planarcontact surfaces 154U, 154L and a peripheral edge 154E. The varistorwafer 156 has first and second opposed, substantially planar contactsurfaces 156U, 156L and a peripheral edge 156E.

The varistor material may be any suitable material conventionally usedfor varistors, namely, a material exhibiting a nonlinear resistancecharacteristic with applied voltage. Preferably, the resistance becomesvery low when a prescribed voltage is exceeded. The varistor materialmay be a doped metal oxide or silicon carbide, for example. Suitablemetal oxides include zinc oxide compounds.

Each varistor wafer 152, 154, 156 may include a wafer of varistormaterial coated on either side with a conductive coating 157 so that theexposed surfaces of the coatings serve as the contact surfaces 152U,152L, 154U, 154L, 156U, 156L. The coatings can be metallization formedof aluminum, copper or silver, for example. Alternatively, the baresurfaces of the varistor material may serve as the contact surfaces152U, 152L, 154U, 154L, 156U, 156L.

The interconnect members 160, 162 are electrically conductive. Theinterconnect member 160 includes a pair of axially spaced apart,disk-shaped contact portions 160U, 160L joined by a bridge portion 160B.The interconnect member 162 includes a pair of axially spaced apart,disk-shaped contact portions 162U, 162L joined by a bridge portion 162B.

According to some embodiments, each contact portion 160U, 160L, 162U,162L is substantially planar, relatively thin and wafer- or disk-shaped.In some embodiments, each contact portion 160U, 160L, 162U, 162L has adiameter D2 (FIG. 8) to thickness T2 (FIG. 8) ratio of at least 15. Insome embodiments, the thickness T2 of each contact portion 160U, 160L,162U, 162L is in the range of from about 0.1 to 3 mm. In someembodiments, the diameter D2 of each contact portion 160U, 160L, 162U,162L is in the range of from about 20 to 100 mm.

According to some embodiments, each contact portion 160U, 160L, 162U,162L does not have any through holes extending through the thickness ofthe contact portion.

In some embodiments, the width W3 (FIG. 6) of each bridge portion 160B,162B is in the range of from about 2 mm to 10 mm. The cross-sectionalarea of each bridge portion 160B, 162B should be large enough towithstand the short circuit current that may flow through the SPD aftera possible failure of one or more of the varistor wafers 152, 154, 156.

According to some embodiments, the interconnect members 160, 162 areformed of copper. However, any suitable electrically conductive metalmay be used. According to some embodiments, the interconnect members160, 162 are unitary and, in some embodiments, monolithic.

In the varistor assembly 150, the varistor wafer 154 is interposed orsandwiched between the varistor wafers 152, 156, the varistor wafers152, 154, 156 are interposed or sandwiched between the interconnectmembers 160, 162, and the interconnect members 160, 162 are interleavedwith one another as shown in FIGS. 6 and 8. The contact portion 160Uengages the contact surface 152U. The contact portion 160L engages thecontact surfaces 154L and 156U. The contact portion 162U engages thecontact surfaces 152L and 154U. The contact portion 162L engages thecontact surface 156L. Each said engagement forms an intimate physical ormechanical contact between the identified interconnect member contactportions and varistor contact surfaces. Each said engagement forms adirect electrical connection or coupling between the identifiedinterconnect member contact portions and varistor contact surfaces. Thecontact portions 160U and 162L form or serve as the outer electrodecontact surfaces of the varistor assembly 150.

Each bridge portion 160B, 162B includes a pair of tab sections 163(extending radially outwardly from the contact portions 160U, 160L or162, 162L) and an axially extending connecting section 165 connectingthe tab sections 163 and radially spaced apart from the adjacentperipheral edges of the varistor wafers 152, 154, 156. In someembodiments, each connecting section 165 is located a distance D3 (FIG.8) from the adjacent peripheral edges of the varistor wafers 152, 154,156. In some embodiments, the distance D3 is in the range of from about0.5 to 15 mm.

According to some embodiments and as shown, there are no electricalinsulators interposed between the components 152, 154, 156, 160, 162.

In some embodiments, the varistor wafers 152, 154, 156 are secured toone another by the bonding agent 164. According to some embodiments, thebonding agent 164 is located at and secures the adjacent varistor wafers152, 154, 156 at their peripheral edges. In some embodiments, thebonding agent 164 is provided as a plurality of discrete, spaced apartpatches or spots of the bonding agent 164. The bonding is used to keepthe components of the varistor assembly 150 in place duringtransportation and assembly of the overvoltage protection device 100.

In some embodiments and as shown in FIGS. 5, 6 and 7, the bonding agent164 includes a bonding agent portion or portions 164′ located within thebridge portions 160, 162B between each bridge portion 160B, 162B and theadjacent edges of the varistor wafers 152, 154, 156. In this way, thesebonding agent portions 164′ can serve as electrical insulators thatelectrically insulate the bridge portions 160B, 162B from the edges ofthe varistor wafers 152, 154, 156.

According to some embodiments, the bonding agent 164 is an adhesive. Asused herein, adhesive refers to adhesives and glues derived from naturaland/or synthetic sources. The adhesive is a polymer that bonds to thesurfaces to be bonded (e.g., the edge surfaces of the varistor wafers152, 154, 156). The adhesive may be any suitable adhesive. In someembodiments, adhesive 164 is secures the varistor wafers 152, 154, 156at their peripheral edges and are discrete, spaced apart patches orspots located about the peripheral edges.

In some embodiments, the adhesive 164 is a cyanoacrylate-based adhesiveor an epoxy-based adhesive. Suitable cyanoacrylate adhesives may includePermabond 737 adhesive available from Permabond Engineering Adhesives,Inc. of the United States of America.

In some embodiments, the adhesive has a high operating temperature,above 40° C., does not contain any solvent, and has a high dielectricstrength (e.g., above 5 kV/mm).

In some embodiments, the outer periphery of each coating 157 is radiallyinset from the outer periphery of the varistor wafer 152, 154, 156, andthe outer periphery of each contact portion 160U, 160L, 162U, 162L isradially inset from the outer periphery of the coating 157.

In other embodiments, the varistor wafers 152, 154, 156 are mechanicallysecured and electrically directly connected to the respective contactportions 160U, 160L, 162U, 162L by an electrically conductive solder.

The varistor assembly 150 can be assembled as follows in accordance withembodiments of the invention.

The interconnect members 160, 162 may be pre-bent into the shapes shownin FIG. 7.

In some embodiments, each contact portion 160U, 160L, 162U, 162L coversand engages at least 40% of the surface area of the corresponding matingvaristor wafer surface 152U, 152L, 154U, 154L, 156U, 156L.

The varistor wafers 152, 154, 156 and the interconnect members 160, 162are stacked and interleaved in the order and relation as shown in FIGS.6 and 8. This assembly may be assembled in or placed, after assembly, ina fixture to laterally align the varistor wafers 152, 154, 156 and theinterconnect members 160, 162 with respect to one another. In someembodiments, the varistor wafers 152, 154, 156 and the interconnectmembers 160, 162 are substantially coaxially aligned.

The aligned components 152, 154, 156, 160, 162 are axially compressivelyloaded, pressed or clamped together (e.g., using the fixture or anadditional external clamping or loading device) and into intimatecontact. The bonding agent 164 is then applied to the peripheral edges152E, 154E, 156E of the varistor wafers 152, 154, 156 at locations asdiscussed above, and cured. The varistor assembly 150 is thus formed.Once the bonding agent 164 has cured, the external loading device isremoved from the varistor assembly 150.

The insulator sleeve 134 is tubular and generally cylindrical. Accordingto some embodiments, the insulator sleeve 134 is formed of a hightemperature polymer and, in some embodiments, a high temperaturethermoplastic. In some embodiments, the insulator sleeve 134 is formedof polyetherimide (PEI), such as ULTEM™ thermoplastic available fromSABIC of Saudi Arabia. In some embodiments, the insulator member 134 isformed of non-reinforced polyetherimide.

According to some embodiments, the insulator sleeve 134 is formed of amaterial having a melting point greater than the melting point of themeltable member 132. According to some embodiments, the insulator sleeve134 is formed of a material having a melting point in the range of fromabout 120 to 200° C.

According to some embodiments, the insulator sleeve 134 material canwithstand a voltage of 25 kV per mm of thickness.

According to some embodiments, the insulator sleeve 134 has a thicknessin the range of from about 0.1 to 2 mm.

The spring washers 128E surround the shaft 124B. Each spring washer 128Eincludes a hole that receives the shaft 124B. The lowermost springwasher 128E abuts the top face of the head 124A. According to someembodiments, the clearance between the spring washer hole and the shaft124B is in the range of from about 0.015 to 0.035 inch. The springwashers 128E may be formed of a resilient material. According to someembodiments and as illustrated, the spring washers 128E are wave washers(as shown) or Belleville washers formed of spring steel. While twospring washers 128E are shown, more or fewer may be used. The springsmay be provided in a different stack arrangement such as in series,parallel, or series and parallel.

The flat metal washer 128D is interposed between the uppermost springwasher 128E and the insulator ring 128C with the shaft 124B extendingthrough a hole formed in the washer 128D. The washer 128D serves todistribute the mechanical load of the upper spring washer 128E toprevent the spring washer 128E from cutting into the insulator ring128C.

The insulator ring 128C overlies and abuts the washer 128D. Theinsulator ring 128C has a main body ring and a cylindrical upper flangeor collar extending upwardly from the main body ring. A hole receivesthe shaft 124B. According to some embodiments, the clearance between thehole and the shaft 124B is in range of from about 0.025 to 0.065 inch.An upwardly and outwardly opening peripheral groove is formed in the topcorner of the main body ring.

The insulator ring 128C is preferably formed of a dielectric orelectrically insulating material having high melting and combustiontemperatures. The insulator ring 128C may be formed of polycarbonate,ceramic or a high temperature polymer, for example.

The end cap 128A overlies and abuts the insulator ring 128C. The end cap128A has a hole that receives the shaft 124B. According to someembodiments, the clearance between the hole and the shaft 124B is in therange of from about 0.1 to 0.2 inch. The end cap 128A may be formed ofaluminum, for example.

The clip 128B is resilient and truncated ring shaped. The clip 128B ispartly received in the slot 12211 and partly extends radially inwardlyfrom the inner wall of the housing 122 to limit outward axialdisplacement of the end cap 128A. The clip 128B may be formed of springsteel.

The O-ring 130B is positioned in the groove 124G so that it is capturedbetween the shaft 124B and the insulator ring 128C. The O-ring 130A ispositioned in the groove in the insulator ring 128C such that it iscaptured between the insulating member 128C and the sidewall 122B. Wheninstalled, the O-rings 130A, 130B are compressed so that they are biasedagainst and form a seal between the adjacent interfacing surfaces. In anovervoltage or failure event, byproducts such as hot gases and fragmentsfrom the varistor wafers 152, 154, 156 may fill or scatter into thecavity chamber 126. These byproducts may be constrained or prevented bythe O-rings 130A, 130B from escaping the overvoltage protection device100 through the housing opening 122D.

The O-rings 130A, 130B may be formed of the same or different materials.

According to some embodiments, the O-rings 130A, 130B are formed of aresilient material, such as an elastomer. According to some embodiments,the O-rings 130A, 130B are formed of rubber. The O-rings 130A, 130B maybe formed of a fluorocarbon rubber such as VITON™ available from DuPont.Other rubbers such as butyl rubber may also be used. According to someembodiments, the rubber has a durometer of between about 60 and 100Shore A.

The electrode head 124A and the housing end wall 122A are persistentlybiased or loaded against the varistor assembly 150 along a load orclamping axis C-C (FIG. 5) in directions F to ensure firm and uniformengagement between the above-identified interfacing contact surfaces.This aspect of the unit 100 may be appreciated by considering a methodaccording to the present invention for assembling the unit 100, asdescribed below. In some embodiments, the clamping axis C-C issubstantially coincident with the axis A-A (FIG. 5).

The components 152, 154, 156, 160, 162, 164 are assembled as describedabove to form the varistor assembly 150. The varistor assembly 150 isplaced in the cavity 122C such that the lower contact surface or portion162L of the interconnect member 162 engages the contact surface 122G ofthe end wall 122A.

The O-rings 130A, 130B are installed in their respective grooves.

The head 124A is inserted into the cavity 122C such that the contactsurface 124C engages the upper contact surface or portion 160U of theinterconnect member 160.

The spring washers 128E are slid down the shaft 124B. The washer 128D,the insulator ring 128C, and the end cap 128A are slid down the shaft124B and over the spring washers 128E. A jig (not shown) or othersuitable device is used to force the end cap 128A down, in turndeflecting the spring washers 128E. While the end cap 128A is stillunder the load of the jig, the clip 128B is compressed and inserted intothe slot 12211. The clip 128B is then released and allowed to return toits original diameter, whereupon it partly fills the slot and partlyextends radially inward into the cavity from the slot 12211. The clip128B and the slot 12211 thereby serve to maintain the load on the endcap 128A to partially deflect the spring washers 128E. The loading ofthe end cap 128A onto the insulator ring 128C and from the insulatorring onto the spring washers is in turn transferred to the head 124A. Inthis way, the varistor assembly 150 is sandwiched (clamped) between thehead 124A and the electrode wall 122A.

When the overvoltage protection device 100 is assembled, the housing122, the electrode 124, the insulating member 128C, the end cap 128A,the clip 128B, and the O-rings 130A, 130B collectively form a unithousing or housing assembly 121 containing the components in the chamber126.

In the assembled overvoltage protection device 100, the large, planarcontact surfaces of the components 122A, 124A, 152, 154, 156, 160, 162can ensure reliable and consistent electrical contact and connectionbetween the components during an overvoltage or surge current event. Thehead 124A and the end wall 122A are mechanically loaded against thesecomponents to ensure firm and uniform engagement between the matingcontact surfaces.

Advantageously, the overvoltage protection device 100 integrates threevaristor wafers 152, 154, 156 in electrical parallel in the same modulardevice, so that energy can be shared between the varistor wafers 152,154, 156 during electrical conduction.

The design of the overvoltage protection device 100 provides compressiveloading of the varistor wafers 152, 154, 156 in a single modular unit.The overvoltage protection device 100 provides suitable electricalinterconnections between the electrodes 122, 124 and the varistor wafers152, 154, 156, while retaining a compact form factor and providingproper thermal dissipation of energy from the varistor wafers 152, 154,156.

The construction of the overvoltage protection device 100 provides asafe failure mode for the device. During use, one or more of thevaristor wafers 152, 154, 156 may be damaged by overheating and maygenerate arcing inside the housing assembly 121. The housing assembly121 can contain the damage (e.g., debris, gases and immediate heat)within the overvoltage protection device 100, so that the overvoltageprotection device 100 fails safely. In this way, the overvoltageprotection device 100 can prevent or reduce any damage to adjacentequipment (e.g., switch gear equipment in the cabinet) and harm topersonnel. In this manner, the overvoltage protection device 100 canenhance the safety of equipment and personnel.

Additionally, the overvoltage protection device 100 provides a fail-safemechanism in response to end of life mode in one of more of the varistorwafers 152, 154, 156. In case of a failure of a varistor wafer 152, 154,156, a fault current will be conducted between the corresponding lineand the neutral line. As is well known, a varistor has an innate nominalclamping voltage VNOM (sometimes referred to as the “breakdown voltage”or simply the “varistor voltage”) at which the varistor begins toconduct current. Below the VNOM, the varistor will not pass current.Above the VNOM, the varistor will conduct a current (i.e., a leakagecurrent or a surge current). The VNOM of a varistor is typicallyspecified as the measured voltage across the varistor with a DC currentof 1 mA.

As is known, a varistor has three modes of operation. In a first normalmode (discussed above), up to a nominal voltage, the varistor ispractically an electrical insulator. In a second normal mode (alsodiscussed above), when the varistor is subjected to an overvoltage, thevaristor temporarily and reversibly becomes an electrical conductorduring the overvoltage condition and returns to the first modethereafter. In a third mode (the so-called end of life mode), thevaristor is effectively depleted and becomes a permanent, non-reversibleelectrical conductor.

The varistor also has an innate clamping voltage VC (sometimes referredto as simply the “clamping voltage”). The clamping voltage VC is definedas the maximum voltage measured across the varistor when a specifiedcurrent is applied to the varistor over time according to a standardprotocol.

In the absence of an overvoltage condition, the varistor wafer 152, 154,156 provides high resistance such that no current flows through theovervoltage protection device 100 as it appears electrically as an opencircuit. That is, ordinarily the varistor passes no current. In theevent of an overcurrent surge event (typically transient; e.g.,lightning strike) or an overvoltage condition or event (typically longerin duration than an overcurrent surge event) exceeding VNOM, theresistance of the varistor wafer decreases rapidly, allowing current toflow through the overvoltage protection device 100 and create a shuntpath for current flow to protect other components of an associatedelectrical system. Normally, the varistor recovers from these eventswithout significant overheating of the overvoltage protection device100.

Varistors have multiple failure modes. The failure modes include: 1) thevaristor fails as a short circuit; and 2) the varistor fails as a linearresistance. The failure of the varistor to a short circuit or to alinear resistance may be caused by the conduction of a single ormultiple surge currents of sufficient magnitude and duration or by asingle or multiple continuous overvoltage events that will drive asufficient current through the varistor.

A short circuit failure typically manifests as a localized pinhole orpuncture site (herein, “the failure site”) extending through thethickness of the varistor. This failure site creates a path for currentflow between the two electrodes of a low resistance, but high enough togenerate ohmic losses and cause overheating of the device even at lowfault currents. Sufficiently large fault current through the varistorcan melt the varistor in the region of the failure site and generate anelectric arc.

A varistor failure as a linear resistance will cause the conduction of alimited current through the varistor that will result in a buildup ofheat. This heat buildup may result in catastrophic thermal runaway andthe device temperature may exceed a prescribed maximum temperature. Forexample, the maximum allowable temperature for the exterior surfaces ofthe device may be set by code or standard to prevent combustion ofadjacent components. If the leakage current is not interrupted at acertain period of time, the overheating will result eventually in thefailure of the varistor to a short circuit as defined above.

In some cases, the current through the failed varistor could also belimited by the power system itself (e.g., ground resistance in thesystem or in photo-voltaic (PV) power source applications where thefault current depends on the power generation capability of the systemat the time of the failure) resulting in a progressive build up oftemperature, even if the varistor failure is a short circuit. There arecases where there is a limited leakage current flow through the varistordue to extended in time overvoltage conditions due to power systemfailures, for example. These conditions may lead to temperature build upin the device, such as when the varistor has failed as a linearresistance and could possibly lead to the failure of the varistor eitheras a linear resistance or as a short circuit as described above.

As discussed above, in some cases the overvoltage protection device 100may assume an “end of life” mode in which a varistor wafer 152, 154, 156is depleted in full or in part (i.e., in an “end of life” state),leading to an end of life failure. When the varistor reaches its end oflife, the overvoltage protection device 100 will become substantially ashort circuit with a very low but non-zero ohmic resistance. As aresult, in an end of life condition, a fault current will continuouslyflow through the varistor even in the absence of an overvoltagecondition. In this case, the meltable member 132 can operate as afail-safe mechanism that by-passes the failed varistor and creates apermanent low-ohmic short circuit between the terminals of theovervoltage protection device 100 in the manner described in U.S. Pat.No. 7,433,169, the disclosure of which is incorporated herein byreference.

The meltable member 132 is adapted and configured to operate as athermal disconnect to electrically short circuit the current applied tothe associated overvoltage protection device 100 around the varistorwafers 152, 154, 156 to prevent or reduce the generation of heat in thevaristors. In this way, the meltable member 132 can operate as switch tobypass the varistor wafers 152, 154, 156 and prevent overheating andcatastrophic failure as described above. As used herein, a fail-safesystem is “triggered” upon occurrence of the conditions necessary tocause the fail-safe system to operate as described to short circuit theelectrodes 122A, 124A.

When heated to a threshold temperature, the meltable member 132 willflow to bridge and electrically connect the electrodes 122A, 124A. Themeltable member 132 thereby redirects the current applied to theovervoltage protection device 100 to bypass the varistors 152, 154, 156so that the current induced heating of the varistor ceases. The meltablemember 132 may thereby serve to prevent or inhibit thermal runaway(caused by or generated in a varistor 152, 154, 156) without requiringthat the current through the overvoltage protection device 100 beinterrupted.

More particularly, the meltable member 132 initially has a firstconfiguration as shown in FIG. 5 such that it does not electricallycouple the electrode 124 and the housing 122 except through the head124A. Upon the occurrence of a heat buildup event, the electrode 124 isthereby heated. The meltable member 132 is also heated directly and/orby the electrode 124. During normal operation, the temperature in themeltable member 132 remains below its melting point so that the meltablemember 132 remains in solid form. However, when the temperature of themeltable member 132 exceeds its melting point, the meltable member 132melts (in full or in part) and flows by force of gravity into a secondconfiguration different from the first configuration. The meltablemember 132 bridges or short circuits the electrode 124 to the housing122 to bypass the varistor wafers 152, 154, 156. That is, a new directflow path or paths are provided from the surface of the electrode 124 tothe surface of the housing sidewall 122B through the meltable member132. According to some embodiments, at least some of these flow paths donot include the varistor wafers 152, 154, 156.

According to some embodiments, the overvoltage protection device 100 isadapted such that when the meltable member 132 is triggered to shortcircuit the overvoltage protection device 100, the conductivity of theovervoltage protection device 100 is at least as great as theconductivity of the feed and exit cables connected to the device.

Electrical protection devices according to embodiments of the presentinvention may provide a number of advantages in addition to thosementioned above. The devices may be formed so to have a relativelycompact form factor. The devices may be retrofittable for installationin place of similar type surge protective devices not having circuits asdescribed herein. In particular, the present devices may have the samelength dimension as such previous devices.

There are applications when there is a requirement for an SPD having alower residual voltage at the same nominal operating voltage. Forexample, this is a requirement for some telecom applications rated for−48 Vdc systems. If an SPD is used that includes a varistor (e.g., anMOV), a typical continuous operation voltage Vc for such a varistor is100 Vdc. However, this SPD will have a residual voltage Vres of around300V or more. It would be beneficial for the better protection of theequipment to use SPDs with a residual voltage Vres much lower than theselevels (i.e., close to 100V).

Typically, in order to reduce the residual voltage of an SPD,manufacturers have used a technology other than varistors, such as SADsor TVS diodes. These components have a much lower residual voltage thanMOVs for the same continuous operating voltage Vc. For example, a TVSdiode for this application may have a residual voltage of 100 V. ButSADs and TVS diodes typically cannot conduct the surge currents ofsignificant energies that are expected in such applications. For thatreason, many manufacturers have used multiple SADs and/or TVS diodes inparallel to achieve higher energy withstand capabilities during surgecurrent conduction.

In the overvoltage protection device 100, the varistor wafers 152, 154,156 are connected in electrical parallel to reduce the residual voltageVres of the overvoltage protection device 100.

In some embodiments, each varistor wafer 152, 154, 156 is rated for 60Vdc (continuous operating voltage; Vc) instead of 100 Vdc that istypical for this application. Further, the use of three varistors inparallel reduces even further the clamping voltage of the SPD at a givensurge current (as compared to using a single varistor), as each varistorwill conduct only a fraction of the overall surge current (the clampingvoltage depends on the conducted surge current, the higher the conductedsurge current the higher the clamping voltage of the varistor). For thetelecom applications (nominal voltage of −48 Vdc), the resultantresidual voltage is around 140 V at a surge current of 5 kA.

In some embodiments, the overvoltage protection device 100 is used in aDC power system and, in some embodiments, in a protection circuit of −48Vdc telecommunications equipment. The device 100 may also be used in ACor other DC applications.

The reduction of the rated voltage of the varistor wafers 152, 154, 156makes the varistor wafers 152, 154, 156 thinner and sensitive tosignificant temperature variations. Therefore, how the stack of varistorwafers is held in place and assembled inside the overvoltage protectiondevice 100 is important.

As mentioned above, in some embodiments the varistor wafers varistorwafers 152, 154, 156 may be secured to the interconnect members 160, 162and/or each other using solder. However, the use of solder may damagethe varistor wafer. The high temperature required to melt the solderingmaterial and the different coefficients of elasticity between thevaristor material and the solder may create micro cracks in thevaristor. Loading on the varistor wafer by electrodes may also causecracks in the varistor wafer. These cracks as well as flux or impuritiesthat intrude into the cracks can progressively damage and thereby deratethe varistor. Intruding flux may create a conductive path on the edge ofa crack that increases leakage current, which can lead to failure of thevaristor wafer. These risks are particularly of concern in the case ofrelatively thin (e.g., less than about 2 mm) ceramic varistor wafers.

Further, to avoid mechanical damage on the varistor due to differentthermal expansion between the varistor and the interconnect members 160,162, the shape of the interconnect member contact portions should beround with a hole in the middle. The hole may decrease the uniformdistribution of the current over the surface of the varistor. The holemay also reduce the energy withstand capability of the varistor duringsurge currents, as it will significantly decrease the heat shrinkcapabilities of the varistor and increase the contact resistance andoverall strength of the stack forming the varistor assembly 150.

As discussed above, in some embodiments, the varistor wafers 152, 154,156 are stacked in parallel and bonded or adhered together by adhesive164 on their edges 152E, 154E, 156E. The adhesive 164 on the edges 152E,154E, 156E provides a compact assembly for transport and manipulation inproduction of the varistor assembly 150 and the device 100.

Moreover, the adhesive 164 rectifies the above mentioned issues. Theadhesive holds the varistor wafers 152, 154, 156 and the interconnectmembers 160, 162 together for handling without introducing heating,solder and flux that may cause micro cracks and introduce conductivepaths as discussed above.

The adhesive permits the use of the contact portions 160U, 160L, 162U,162L of the interconnect members that do not include holes within theirperipheries (i.e., are full face electrodes). As a result, the energywithstand capability of the varistor assembly 150 during surge events isincreased. The contact resistances between the varistor wafers 152, 154,156 and the interconnect members 160, 162 are reduced. The expectedresidual voltage during surges is thereby reduced.

According to some embodiments, the areas of engagement between each ofthe electrode contact surfaces and the varistor contact surfaces areeach at least one square inch.

According to some embodiments, the biased electrodes (e.g., theelectrodes 122 and 124) apply a load to the varistors along the axis C-Cin the range of from 2000 lbf and 26000 lbf depending on its surfacearea.

According to some embodiments, the combined thermal mass of the housing(e.g., the housing 122) and the electrode (e.g., the electrode 124) issubstantially greater than the thermal mass of each of the varistorscaptured therebetween. The greater the ratio between the thermal mass ofthe housing and electrodes and the thermal mass of the varistors, thebetter the varistors will be preserved during exposure to surge currentsand TOV events and therefore the longer the lifetime of the SPD. As usedherein, the term “thermal mass” means the product of the specific heatof the material or materials of the object multiplied by the mass ormasses of the material or materials of the object. That is, the thermalmass is the quantity of energy required to raise one gram of thematerial or materials of the object by one degree centigrade times themass or masses of the material or materials in the object. According tosome embodiments, the thermal mass of at least one of the electrode headand the electrode wall is substantially greater than the thermal mass ofthe varistor. According to some embodiments, the thermal mass of atleast one of the electrode head and the electrode wall is at least twotimes the thermal mass of the varistor, and, according to someembodiments, at least ten times as great. According to some embodiments,the combined thermal masses of the head and the electrode wall aresubstantially greater than the thermal mass of the varistor, accordingto some embodiments at least two times the thermal mass of the varistorand, according to some embodiments, at least ten times as great.

As discussed above, the spring washers 128E are Belleville or wavewashers. Belleville or wave washers may be used to apply relatively highloading without requiring substantial axial space. However, other typesof biasing means may be used in addition to or in place of theBelleville or wave washers. Suitable alternative biasing means includeone or more coil springs or spiral washers.

The varistor assembly 150 includes three varistors and two interconnectmembers. However, varistor assemblies according to further embodimentsmay include more than three varistors stacked and connected inelectrical parallel as described. For example, a varistor assembly caninclude five varistors stacked and connected in electrical parallel bythree interconnect members.

With reference to FIGS. 10-12, a modular overvoltage protection unit 200according to further embodiments of the invention is shown therein. Theovervoltage protection unit 200 can be used in the same manner and forthe same purpose as the overvoltage protection device 100, except thatthe unit 200 is generally operationally equivalent to two if theovervoltage protection devices 100.

The overvoltage protection unit 200 includes a housing assembly 221 andtwo SPD internal component sets or submodules 211, 212.

The housing assembly 221 includes a first electrode or housing 223 and acover 226. The housing 223 is unitary and, in some embodiments,monolithic. The housing 223 is formed of an electrically conductivemetal such as aluminum. The housing 223 includes two integral housingelectrode wall portions 222. Each housing electrode portion 222 includesan electrode wall 222A, a sidewall 222B, a cavity 222C, and a topopening 222D corresponding to the features 122A, 122B, 122C and 122D,respectively, of the device 100.

The cover 226 is substantially plate-shaped and has a profile matchingthat of the housing 223. The cover 226 has two electrode openings 226Aand six fastening bores 226B defined therein. According to someembodiments, the cover 226 is formed of an electrically conductivematerial. In some embodiments, the cover 226 is formed of a metal and,in some embodiments, are formed of aluminum.

The SPD submodules 211, 212 each include an electrode 224, a meltablemember 232, an insulator sleeve 234, and a varistor assembly 250corresponding to the components 124, 132, 134, and 150, respectively, ofthe device 100. Each SPD submodule 211, 212 further includes anelastomeric insulator member 239.

The insulator members 239 are formed of an electrically insulating,resilient, elastomeric material. According to some embodiments, theinsulator members 239 are formed of a material having a hardness in therange of from about 60 Shore A to 85 Shore A. According to someembodiments, the insulator members 239 are formed of rubber. Accordingto some embodiments, the insulator members 239 are formed of siliconerubber. Suitable materials for the insulator members 239 may includeKE-5612G or KE-5606 silicone rubber available from Shin-Etsu ChemicalCo. Ltd.

Each SPD submodule 211, 212 is disposed in respective one of the housingcavities 222C. The cover 226 is secured to the housing 223 by bolts 5.The cover 226 captures the SPD submodules 211, 212 and axiallycompresses the elastomeric insulators 239 thereof. The SPD submodule 211and its electrode wall 222A form a first overvoltage protection devicecorresponding to the device 100. The SPD submodule 212 and its electrodewall 222A form a second overvoltage protection device corresponding tothe device 100.

When the unit 200 is assembled, the insulator member 239 of each SPDsubmodule 211, 212 is captured between the cover 226 and the electrodeupper flange 224D and axially compressed (i.e., axially loaded andelastically deformed from its relaxed state) so that the insulatormember 239 serves as a biasing member and applies a persistent axialpressure or load to the electrode 224 and the cover 226. The insulatormember 239 also serves to electrically insulate the housing 223 from theelectrode 224. The compressed insulator member 239 can also form a sealto constrain or prevent overvoltage event byproducts, such as hot gasesand fragments from the varistor wafers of the varistor assembly 250 fromescaping the unit 200 through the corresponding housing opening 222D.

The varistor assemblies 250 can provide the same advantages in the unit200 as discussed above for the varistor assembly 150. Each varistorassembly 250 includes adhesive 264 corresponding to the adhesive 164,164′.

In other embodiments, the SPD submodules 211, 212 can employ separatesprings and insulating rings as described with regard to the device 100.

In further embodiments, each SPD submodule 211, 212 can include a singlevaristor wafer in place of the multi-varistor varistor assembly 250.

With reference to FIG. 13, a modular overvoltage protection device 300according to further embodiments of the invention is shown therein. Theovervoltage protection unit 300 can be used in the same manner and forthe same purpose as the overvoltage protection device 100. Theovervoltage protection device 300 is constructed in the same manner asthe overvoltage protection device 100, except as follows.

The overvoltage protection device 300 includes a varistor assembly 350corresponding to the varistor assembly 150, except as follows. Thevaristor assembly 350 includes five varistor wafers 352, 353, 354, 355,356, four interconnect members 360, 362, 366, 368, and bonding agents364. The varistor wafers 352, 353, 354, 355, 356 correspond to and areformed in the same manner as the varistor wafers 152, 154, 156. Theinterconnect members 360, 362, 366, 368 correspond to and are formed inthe same manner as the interconnect members 160, 162. The bonding agents364 correspond to the bonding agents 164, 164′. The five varistor wafers352, 353, 354, 355, 356 are axially stacked, bonded and connected inelectrical parallel by the four interconnect members 360, 362, 366, 368.

With reference to FIG. 14, a modular overvoltage protection unit 400according to further embodiments of the invention is shown therein. Theovervoltage protection device 400 can be used in the same manner and forthe same purpose as the overvoltage protection device 100. Theovervoltage protection device 300 is constructed in the same manner asthe overvoltage protection device 100, except as follows.

The overvoltage protection device 400 includes a varistor assembly 450corresponding to the varistor assembly 150, except as follows. Thevaristor assembly 450 includes two varistor wafers 452, 454, twointerconnect members 460, 462, bonding agents 464 and an electricalinsulator wafer 457. The varistor wafers 452, 454 correspond to and areformed in the same manner as the varistor wafers 152, 154, 156. Theinterconnect members 460, 462 correspond to and are formed in the samemanner as the interconnect members 160, 162. The bonding agents 464correspond to the bonding agents 164, 164′. The insulator wafer 457 isformed of an electrically insulating material. Suitable electricalinsulating materials may include ULTEM™ 1000 thermoplastic availablefrom SABIC, mica, or polyester film such as DYFILM™ polyester filmavailable from Coveme of Italy, for example. The two varistor wafers452, 454 are axially stacked and connected in electrical parallel by thetwo interconnect members 460, 462. The insulator wafer 457 is axiallyinterposed or stacked between the varistor wafers 452, 454 to preventshort circuiting between the opposing faces of the varistor wafers 452,454.

With reference to FIGS. 15 and 16, a modular overvoltage protectiondevice 500 according to further embodiments of the invention is showntherein. The overvoltage protection device 500 can be used in the samemanner and for the same purpose as the overvoltage protection device100.

The overvoltage protection device 500 is constructed as one half of theunit 200 (FIG. 12). The device 500 includes a housing assembly 521 thatis one half of the housing assembly 221 and an SPD internal componentset 512 corresponding to the submodule 212.

With reference to FIGS. 17-20, a modular overvoltage protection device600 according to further embodiments of the invention is shown therein.The overvoltage protection device 600 can be used in the same manner andfor the same purpose as the overvoltage protection device 100. Theovervoltage protection device 600 is constructed in the same manner asthe overvoltage protection device 100, except as follows.

The overvoltage protection device 600 includes a varistor assembly 650corresponding to the varistor assembly 150, except as follows. Thevaristor assembly 650 includes three varistor wafers 652, 654, 656 andtwo interconnect members 660, 662. The varistor wafers 652, 654, 656correspond to and are formed in the same manner as the varistor wafers152, 154, 156. The interconnect members 660, 662 correspond to and areformed in the same manner as the interconnect members 160, 162. Thevaristor wafers 652, 654, 656 are axially stacked and connected inelectrical parallel by the interconnect members 660, 662 as discussedabove for the device 100.

The overvoltage protection device 600 further includes an electricallyinsulating void filling member or sleeve 636. The sleeve 636 includes aside wall 636A defining a through passage 636B. The passage 636 extendsfrom an upper opening 636C to a lower opening 636D. A pair of laterallyopposing, axially extending receiver channels 636E are defined in theinner surface 636F of the side wall 636A.

The sleeve 636 is tubular and has an outer surface 636G that isgenerally cylindrical. According to some embodiments, the sleeve 636 isformed of a high temperature polymer and, in some embodiments, a hightemperature thermoplastic. In some embodiments, the sleeve 636 is formedof polyetherimide (PEI), such as ULTEM′ thermoplastic available fromSABIC of Saudi Arabia. In some embodiments, the sleeve 636 is formed ofnon-reinforced polyetherimide. In some embodiments, the sleeve 636 isformed of an electrically insulating ceramic.

According to some embodiments, the sleeve 636 is formed of a materialhaving a melting point greater than the melting point of the meltablemember 632. According to some embodiments, the sleeve 636 is formed of amaterial having a melting point in the range of from about 120 to 200°C.

According to some embodiments, the sleeve 636 material can withstand avoltage of 25 kV per mm of thickness.

According to some embodiments, the sleeve side wall 636A has a nominalthickness T5 (FIG. 20) of at least 2 mm, in some embodiments at least 4mm, and in some embodiments in the range of from about 2 to 15 mm.According to some embodiments, the depth D5 of each receiver channel636E is at least 1 mm and, in some embodiments, in the range of fromabout 1 to 12 mm.

The internal chamber 627 of the housing assembly 621 of the overvoltageprotection device 600 includes a first subchamber 627A and a secondsubchamber 627B in fluid communication with the first subchamber 627A.Prior to melting of the meltable member 632, the electrode 624 and themeltable member 632 occupy the first subchamber 627A. The varistorassembly 650 occupies a central volume of the second subchamber 627Bsuch that a remaining tubular void or gap volume 627C of the secondsubchamber 627B remains unoccupied by the varistor assembly 650. The gapvolume 627C is the space or volume extending radially between thevaristor assembly 650 and the inner surface 622H of the sidewall 622B ofthe housing electrode 622. The void filling sleeve 636 occupies the gapvolume 627C and surrounds the varistor assembly 650.

The receiver recesses or channels 636E and the bridge portions 660B,662B of the interconnect members 660, 662 are relatively sized andassembled such that each of the bridge portions 660B, 662B is receivedand seated in a respective one of the receiver channels 636E. Theremainder of the sleeve inner surface 636F generally conforms to theperipheral profiles of the varistor wafers 652, 654, 654.

Thus, as can be appreciated from FIGS. 17 and 19, the inner surface 636Fgenerally conforms to the outer shape of the varistor assembly 650. Thecylindrical outer surface 636G generally conforms to the inner shape ofthe inner wall surface 622H of the housing electrode 622. In someembodiments, the gap between the inner surface 636F and the varistorwafers 652, 654, 654 is less than 2 mm. In some embodiments, the gapbetween the outer surface 636G and the inner wall surface 622H is lessthan 0.5 mm.

The varistor wafers 652, 654, 656 are relatively thick so that theoverall height of the varistor assembly 650 is increased as compared tothat of the varistor assembly 150, for example. As a result, the gapvoid or volume 627C surrounding the varistor assembly 650 is relativelylarge. Additionally, the bridge portions 660B, 662B project radiallyoutwardly beyond the peripheral edges of the varistors 652, 654, 656.Because the inner surface 622H of the housing electrode 622 iscylindrical, the required spacing between the bridge portions 660B, 662Band the inner surface 622B creates relatively large gaps around theremainder of the varistor assembly 650.

In the absence of the void filling sleeve 636, this large gap volume627C could compromise the intended operation of the meltable member 632and the fail-safe mechanism 602. In particular, the volume of the meltedmeltable member 632 may not be sufficient to bridge the electrodes 622and 624 to short circuit the electrodes 622, 624, depending on theorientation of the device 600 when the meltable member 632 is melted.The spacer sleeve 636 occupies the gap volume 627C and thereby reducesor limits the amount or volume of the meltable member 632 that can flowinto the gap volume 627C when the meltable member 632 becomes molten. Inthis way, the void filling member 636 ensures that a greater andreliably sufficient quantity of the melted meltable member is retainedin the first subchamber 627A to make simultaneous contact with the twoelectrodes 622, 624.

In some embodiments, the void filling sleeve 636 occupies at least 50percent of the gap volume 627C and, in some embodiments, at least 80percent. In some embodiments, the void filling sleeve 636 has a volumein the range of from about 100 mm³ to 100,000 mm³ and, in someembodiments, the volume is about 21,000 mm³.

While the illustrated void filling member 636 is configured as aunitary, tubular sleeve having axially extending receiver channels 636Edefined therein, other configurations and constructions may be employed.For example, the channels 636E may be replaced with radially extendingbores that do not extend to the ends of the sleeve. The void fillingmember 636 may be replaced with two or more void filling members thatare configured and arranged to occupy the gap volume 627C to the degreeand with the dimensions discussed above. The two or more void fillingmembers may be axially stacked and or may each surround the varistorassembly 650 by less than 360 degrees.

With reference to FIG. 21, a modular overvoltage protection device 700according to further embodiments of the invention is shown therein. Theovervoltage protection device 700 can be used in the same manner and forthe same purpose as the overvoltage protection device 600. Theovervoltage protection device 700 is constructed in the same manner asthe overvoltage protection device 600, except as follows. The device 700includes a varistor assembly 750 corresponding to the varistor assembly650, and a void filling member 736 corresponding to the void fillingmember 636.

The overvoltage protection device 700 includes an elastomeric insulatormember 739 corresponding to the elastomeric insulator member 239 (FIG.12). The insulator member 739 is captured between the cover 726 and theelectrode upper flange 724D and axially compressed (i.e., axially loadedand elastically deformed from its relaxed state) so that the insulatormember 739 serves as a biasing member and applies a persistent axialpressure or load to the electrode 724 and the cover 726, as describedwith regard to the unit 200.

It will be appreciated that various aspects as disclosed herein can beused in different combinations. For example, an elastomeric insulatormember corresponding to the elastomeric insulator member 239 can be usedon place of the springs and end insulator members (e.g., insulatormember 128C) of the overvoltage protection devices 100, 300, 400, 600.The varistor assemblies of each device 100-700 can be replaced with avaristor assembly of another one of the devices 100-700 (e.g., thefive-wafer varistor assembly 350 or the two-wafer varistor assembly 450can be used in place of the varistor assembly 650 in the device 600).

Many alterations and modifications may be made by those having ordinaryskill in the art, given the benefit of present disclosure, withoutdeparting from the spirit and scope of the invention. Therefore, it mustbe understood that the illustrated embodiments have been set forth onlyfor the purposes of example, and that it should not be taken as limitingthe invention as defined by the following claims. The following claims,therefore, are to be read to include not only the combination ofelements which are literally set forth but all equivalent elements forperforming substantially the same function in substantially the same wayto obtain substantially the same result. The claims are thus to beunderstood to include what is specifically illustrated and describedabove, what is conceptually equivalent, and also what incorporates theessential idea of the invention.

What is claimed:
 1. An overvoltage protection device comprising: a firstelectrode member; a second electrode member; a varistor interposedbetween and electrically connected to each of the first and secondelectrode members; an electrically conductive meltable member, whereinthe meltable member is responsive to heat in the overvoltage protectiondevice to melt and form an electrical short circuit path across thefirst and second electrode members; and a void filling membersurrounding at least a portion of the varistor, wherein the void fillingmember is formed of an electrically insulating material; wherein: theovervoltage protection device includes a sidewall defining a chamber,the chamber including a first subchamber and a second subchamber influid communication with the first subchamber; the meltable member isdisposed in the first subchamber; the varistor is disposed in the secondsubchamber and a gap volume is defined between the varistor and thesidewall in the second subchamber; and the void filling member isdisposed in the gap volume to limit a flow of the meltable member intothe gap volume.
 2. The overvoltage protection device of claim 1 whereinthe void filling member is formed of an electrically insulating ceramic.3. The overvoltage protection device of claim 1 wherein the void fillingmember occupies at least 50 percent of the gap volume.
 4. Theovervoltage protection device of claim 1 wherein the void filling memberincludes a tubular void filling sleeve that surrounds the varistorwafer.
 5. The overvoltage protection device of claim 4 wherein: the voidfilling sleeve includes an inner surface that generally conforms to anouter shape of the varistor; and the void filling sleeve includes anouter surface that generally conforms to the shape of an inner wallsurface of the sidewall.
 6. The overvoltage protection device of claim 5wherein: a gap between the inner surface of the void filling sleeve andthe varistor is less than 2 mm; and a gap between the outer surface ofthe void filling sleeve and the inner wall surface of the sidewall isless than 0.5 mm.
 7. The overvoltage protection device of claim 4wherein the void filling sleeve includes a sleeve side wall having anominal thickness of at least 2 mm.
 8. The overvoltage protection deviceof claim 4 wherein the void filling sleeve is unitary.
 9. Theovervoltage protection device of claim 1 wherein: the varistor is afirst varistor wafer; the overvoltage protection device includes: asecond varistor wafer formed of a varistor material; and an electricallyconductive interconnect member connecting the first and second varistorwafers in electrical parallel between the first and second electrodemembers; wherein the first and second varistor wafers are axiallystacked between the first and second electrode members; the void fillingmember includes a receiver recess; and a portion of the interconnectmember extends outwardly beyond the first and second varistor wafers andis disposed in the receiver recess.
 10. The overvoltage protectiondevice of claim 9 wherein the receiver recess is an axially extendingchannel defined in an inner surface of the void filling member.
 11. Theovervoltage protection device of claim 9 wherein the receiver recess hasa depth in the range of from about 1 to 12 mm.
 12. The overvoltageprotection device of claim 1 wherein the void filling member is formedof a thermoplastic.
 13. The overvoltage protection device of claim 1wherein the void filling member is formed of a material having a meltingpoint in the range of from about 120 to 200° C.
 14. The overvoltageprotection device of claim 1 wherein the void filling member is formedof a material that can withstand a voltage of 25 kV per mm of thickness.15. The overvoltage protection device of claim 1 wherein the voidfilling member has a volume in the range of from about 100 mm³ to100,000 mm³.
 16. The overvoltage protection device of claim 1 wherein:the first electrode member includes a housing electrode including thesidewall and an end wall integral with the sidewall; the sidewall andthe end wall collectively define the chamber; the second electrodemember extends into the chamber; and the housing electrode is unitarilyformed of metal.
 17. The overvoltage protection device of claim 16wherein the meltable member is mounted on the second electrode.
 18. Theovervoltage protection device of claim 1 wherein: the void fillingmember occupies at least 50 percent of the gap volume; the void fillingmember includes a tubular void filling sleeve that surrounds thevaristor wafer; the void filling sleeve includes an inner surface thatgenerally conforms to an outer shape of the varistor; the void fillingsleeve includes an outer surface that generally conforms to the shape ofan inner wall surface of the sidewall; the void filling sleeve includesa sleeve side wall having a nominal thickness of at least 2 mm; and thevoid filling member is formed of an electrically insulating ceramic. 19.The overvoltage protection device of claim 18 wherein: a gap between theinner surface of the void filling sleeve and the varistor is less than 2mm; and a gap between the outer surface of the void filling sleeve andthe inner wall surface of the sidewall is less than 0.5 mm.
 20. Theovervoltage protection device of claim 18 wherein: the first electrodemember includes a housing electrode including the sidewall and an endwall integral with the sidewall; the sidewall and the end wallcollectively define the chamber; the second electrode member extendsinto the chamber; and the housing electrode is unitarily formed ofmetal.